This invention relates to a control device operated in braking a stepping motor to stop at a predetermined position its rotor or a unit mechanically coupled to the rotor.
In general, in the application of a stepping motor, it is necessary to quickly move the rotor or a unit coupled to the rotor to a target position and to accurately stop it there. For instance in the case where the writing and reading head of a stationary disk memory device is driven with a stepping motor, it is necessary for the head to quickly move to an aimed track to reduce the access time, and it is necessary for the head to accurately stop at the track to correctly write and read the data. Driving the stepping motor at high speed is, in general, not compatible with stopping it accurately; that is, it is not always simple to make the two operations compatible with each other. However, it should be noted that a stepping motor is generally of plural phase type, for instance two-phase type, and therefore, when a vector position defined by the phase currents of the two-phase coils is specified, the rotor can be accurately stopped at a predetermined angular position. That is, the rotor of the stepping motor has an angular position corresponding to a vector position defined by the phase currents, so that it is stably locked or held at the position. However, in stopping the rotor which is being rotated at high speed, even if the phase currents have specified a vector position corresponding to an angular position where the rotor should be stably stopped, the rotor oscillates over the position before stopping there; that is, it takes a relatively long period of time for the rotor to finally stop at the aimed position. The high speed rotation of the rotor loses its effect.
Accordingly, in stopping the rotor of the stepping motor which is being rotated at high speed, it is necessary to suitably brake the rotor thereby to prevent its oscillation over the aimed stop position. For this purpose, a negative-phase braking system has been well known in the art. In the system, after a vector position corresponding to the final stop position is specified for the stepping motor with the phase currents, a vector position preceding the above-described vector position by one step is specified so that the rotor of the stepping motor is subjected to negative-phase braking to reduce the speed of the rotor. Thereafter, the initial vector position is specified again so that the rotor is locked at the final stop position.
In the conventional negative-phase braking system, the period of time which elapses from the time instant that the final vector position is specified until the vector position for negative-phase braking is specified, and the period of time which elapses from the time instant that the vector position for negative-phase braking is specified as described above until the final vector position is specified again can be set independently of each other. When these periods of time are set to suitable values, the rotor of the stepping motor is locked at the aimed stop position without oscillation or with one or two oscillations. However, the system is still disadvantageous in the following points: The above-described periods of time are set to fixed values. Therefore, if the mechanical load of the stepping motor changes after setting the periods of time, then the rotor is braked insufficiently or excessively, so that an expected effect is not obtained. Furthermore, the system is considerably sensible to variation of the supply voltage for the stepping motor.